1. Field of the invention
The present invention relates to integrated MOS circuits and particularly to a circuit for driving a power MOS transistor.
2. Description of the Prior Art
The driving of a power MOS transistor under certain circumstances presents problems. This is the case for instance of a power MOS transistor of an output stage of a logic circuit utilized for switching a supply voltage across an externally connected load. In this situation, in order to ensure a complete turning-on (saturation) of the power transistor during an ON period, a relatively high voltage must be maintained between the gate and the source of the transistor (in the order of about 10 Volts). Should a normal bias configuration be used for the power transistor, an excessive voltage drop across the transistor itself would develop.
In these instances, particular circuit techniques are used which allow for the gate voltage to reach absolute levels even higher than the drain voltage of the power transistor. A known practice is to use the so-called "bootstrap" circuit which is essentially a regenerative feedback circuit used for increasing the gate potential with respect to the source (output node) potential. An alternative circuit arrangement of the prior art contemplates the "release" of the gate potential from the drain potential of the output transistor by utilizing a constant voltage generator connected between the output node OUT (source of the power output transistor) and a V.sub.GG node and by supplying the driving device of the power transistor from these circuit nodes. In this way, the gate voltage of the power transistor may assume a value close to the voltage V.sub.GG, which may also be higher than the drain voltage V.sub.DD of the power transistor when the latter is conducting.
An example of a driving circuit of this known type (high side driver) is depicted in FIGS. 1, 2 and 3. The potential difference between the nodes V.sub.SS and V.sub.GG is maintained constant by means of a suitable voltage generator (a battery or a power supply) while the absolute value of the voltage of nodes V.sub.GG and V.sub.SS depends on the state (ON or OFF) of the power transistor M1, which switches a supply voltage V.sub.DD across the load Zc in function of a driving signal C which is applied to the gate of the driver transistor M2, which may be considered the input terminal IN of the driving circuit.
Block 2 of FIG. 1 is essentially an amplifier A, as depicted in FIGS. 2 and 3. In both circuits, the switching OFF and ON of the input transistor M2 causes, by means of the amplifier A, the switching ON and the switching OFF of the power transistor M1. A limiting diode D is commonly added in order to protect the input of the amplifier A when the transistor M2 is ON by preventing the potential at the input of the amplifier A to become eccessively negative with respect to the voltage V.sub.SS.
A current generator I (FIG. 2), or a resistor R.sub.p (FIG. 3) ensures the maintainance of the drain potential of the transistor M2 at a correct logic level (1) when the transistor is OFF.
In general, the driving circuits in accordance with the prior art, have the disadvantage of dissipating energy during the periods when the transistor M2 is ON and the state of the output transistor M1 is tied to the state of the input transistor M2 of the driving circuit.